The present invention relates to xe2x80x9cactive implantable medical devicesxe2x80x9d as defined by the Jun. 20, 1990 Directive No. 90/385/CEE of the Council of the European Communities, more particularly to pacemakers, defibrillators and/or cardiovertors which are capable of delivering low-energy stimulation pulses to the heart for treatment of heart (cardiac) rate disorders. The invention also relates to the prevention of consequences which extrasystoles can have on the operation of such devices.
Extrasystoles, the appearance of an extra spontaneous contraction of a cardiac chamber out of the normal sequence and rhythm, are known. They can be either ventricular in origin (VES) or atrial in origin (AES).
There are two types of VES extrasystoles. The first VES type corresponds to a ventricular detection (i.e., the sensing or xe2x80x9cdetectionxe2x80x9d of a spontaneous ventricular contraction) or ventricular stimulation (i.e., a low-energy stimulation pulse delivered by the device in the ventricle) that is not preceded by an atrial event (i.e., either an atrial detection or a stimulation delivered by the device in the atrium) in an interval of time considered to be physiological. A physiological time interval refers to the time following an atrial event when a ventricular event should occur to be physiologically healthy to a patient, for example, an interval ranging between 31 and 300 ms. A second VES type corresponds to a ventricular detection that is preceded by an atrial event in an interval of time ranging between 31 and 300 ms, if the atrio-ventricular time (i.e., the time occurring between the atrial event and the following ventricular detection) of the examined cycle is at least 31 ms less than the atrio-ventricular time of the preceding cardiac cycle. The xe2x80x9ccardiac cyclexe2x80x9d is defined as the interval of time between two events of a comparable nature in the same cardiac cavity.
An AES-type extrasystole corresponds to a xe2x80x9cP wavexe2x80x9d (also called a xe2x80x9cP eventxe2x80x9d, i.e., a detection of spontaneous activity having its origin in an atrium) following the P wave of the preceding atrial event, within an interval that is less than a fraction of the average interval of the atrial frequency, calculated over an average of a number, e.g., eight, of cardiac cycles not including an extrasystole. The atrial frequency is the cardiac rate as measured based on successive atrial events. The cardiac rate may be measured in terms of a frequency of beats per minute (bpm).
In practice, all patients present a certain number of isolated VES or AES events without suffering any adverse consequence. But when these extrasystoles become too frequent, typically, more than ten VES or AES events per minute, this phenomenon can impair the filling of the heart cavities with blood, and affect the hemodynamic function of the heart. Furthermore, such phenomenon can induce heart-rate disorders.
The frequency at which either AES or VES events are manifested is determined by a parameter known as the xe2x80x9cextrasystole ratexe2x80x9d. This parameter increases when the frequency at which either AES or VES events appear increases.
In certain patients, the appearance of a high extrasystole rate can be related to a heart rate level that is either too low (i.e., a xe2x80x9cbrady-dependent extrasystolexe2x80x9d), or a heart-rate level that is too high (i.e., a xe2x80x9ctachy-dependent extrasystolexe2x80x9d).
The inventors have discovered that, when a stimulation frequency is determined by the device according to the patient""s activity or cardiac output requirements (i.e., the device is an enslaved (also called a rate-responsive) device which determines a stimulation frequency as a function of a physiological and/or a physical parameter indicative of the patient""s level of activity), the appearance of brady-dependent or tachy-dependent extrasystoles can, in certain cases, result in an undesirable phenomenon of oscillation and instability of the stimulation frequency control algorithm. This phenomenon appears most particularly in devices equipped with a control function known as a xe2x80x9crest frequency algorithmxe2x80x9d as described, for example, in EP-A-0 672 433 and the corresponding U.S. Pat. No. 5,645,576, commonly assigned to the assignee hereof ELA Mxc3xa9dical. This algorithm adjusts the base stimulation frequency to the activity of the patient, and, in particular, lowers the base stimulation frequency to a minimum frequency limit in the case of an extended period of rest (i.e., a xe2x80x9crest phasexe2x80x9d) exhibited by the patient. The minimum frequency limit reached during an extended rest period is often called the xe2x80x9csleep frequencyxe2x80x9d.
When the patient is prone to brady-dependent extrasystoles, an xe2x80x9coscillationxe2x80x9d phenomenon can be observed as follows. First, in the case of an extended rest period, the base stimulation frequency of the patient is lowered until the base stimulation frequency reaches the initial sleep frequency. This low frequency then induces extrasystoles. As a result, the rest frequency control algorithm, which has a built-in monitoring function, diagnoses the presence of a sufficiently large extrasystole condition and then increases the base stimulation frequency to above the range that induces extrasystoles. Then, because the device also detects any prolonged rest period, the base stimulation frequency will again be lowered by the rest frequency control algorithm, making it possible to return the stimulation frequency to a level which will again induce extrasystole events.
A similar oscillation phenomenon appears in the effort phase of the patient, with the devices equipped with a stimulation frequency that is determined as a function of patient activity. Such a stimulation frequency control is also referred to herein as an xe2x80x9censlaved frequencyxe2x80x9d. In these devices, the enslaved frequency control algorithm adjusts the stimulation frequency to the activity level of the patient, and, in particular, increases the stimulation frequency with the level of effort up to a maximum frequency limit. If the patient presents a tachy-dependent extrasystole, an acceleration of the stimulation frequency can be a factor favoring the appearance of undesirable extrasystoles. This, in turn, will trigger a monitoring function in the control algorithm which causes the device to return to a lower enslaved frequency, with an ensuing rise in the stimulation frequency due to the patient""s activity level, which again induces extrasystoles, and so on.
It is, therefore, an object of the present invention to provide a device that is capable of detecting the appearance of such undesirable extrasystole oscillation phenomena and to place the device in a state which will inhibit the appearance of such undesirable phenomena.
Broadly, one aspect of the invention is directed to a device of a general type as described, for example, in the EP-A-0 672 433 and corresponding U.S. Pat. No. 5,645,576, which includes: means for determining an activity level of the patient bearing the device, including analyzing the activity and discriminating between a rest phase, a normal activity phase and an effort phase; means for stimulating at least one cardiac cavity, capable of delivering to the heart pulses at a base stimulation frequency determined by the device; and means for adjusting the base stimulation frequency according to the determined activity level, including lowering the base stimulation frequency to an initial base stimulation frequency minimum level limit during a determined rest phase.
To detect brady-dependent extrasystoles and to alleviate the consequences of such brady-dependent extrasystoles, the device also includes: means for detecting the occurrence of extrasystoles, means for evaluating a corresponding rate of occurrence of such extrasystoles, and means for diagnosing a brady-dependent extrasystole condition and increasing the minimum level limit of the base frequency when the extrasystole rate exceeds a predetermined threshold during the detected rest phase.
Advantageously, an increase in the minimum base frequency level is maintained at least for the entire duration of the rest phase, and preferably is not restored to its former or initial value until the detection of an effort phase, subsequent to the rest phase. Thus, extrasystoles are avoided.
Another aspect of the invention is directed to a device of the aforementioned general type which includes means for detecting tachy-dependent extrasystoles and preventing the undesirable effects of such tachy-dependent extrasystoles. Such a device thus includes means for detecting an occurrence of extrasystoles during an effort phase, means for evaluating a corresponding extrasystole rate, and means for diagnosing a tachy-dependent extrasystole condition and decreasing an enslaved frequency maximum level limit when the extrasystole rate exceeds a predetermined threshold during the aforementioned effort phase.
Advantageously, the reduction in the enslaved frequency maximum level is maintained at least for the entire duration of the aforementioned effort phase, and is preferably not restored to its former or its initial value until the detection of a rest phase, subsequent to the aforementioned effort phase.